With the recent need for higher functionality and smaller and lighter electronic products, the trend toward higher integration and higher packing density of electronic components has been accelerating, and semiconductor packages used in these electronic products have been shrinking in size and thickness more than ever before (for example, refer to cited document 1).
With the trend toward smaller semiconductor packages, the wiring circuit density has been increasing, and through-holes for providing electrical connections between the upper and lower surfaces of the internal layers have come to be formed at higher density and with smaller diameter. Since there is a limit to how small the through-holes can be formed by a conventional method using a mechanical drill, laser drilling has come to be employed in recent years in place of mechanical drilling (for example, refer to cited document 2). Using a CO2 laser or a UV-YAG laser, through-holes can be drilled to a diameter smaller than 100 μm which was the smallest diameter that was possible with conventional mechanical drilling.
Further, in the case of mechanical drilling, when drilling through-holes as small as about 100 μm, the blade of the drill can easily break, tending to cause a problem in drilling; in contrast, with laser drilling which does not use any consumable parts, such through-holes can be efficiently drilled. Furthermore, since smaller-diameter through-holes can be drilled, the spacing of the through-holes can be reduced, and as a result, the size of the printed wiring board can be reduced.
Further, since equipment such as a CO2 laser drill and a UV-YAG laser drill is already in widespread use for drilling interlayer connecting microvias in printed wiring boards, there is often no need to newly design a laser drill for through-hole drilling.